Solve[ForAll[{x, y},
Equivalent[
m (x/a) + n (y/b) ==
1, ((t1 - t2) (b (-1 + t1 t2) x +
a (b + b t1 t2 - (t1 + t2) y)))/((1 + t1^2) (1 + t2^2)) ==
0]], {m, n}, Reals] // Simplify

The code generates an answer containing “if”. How can I remove the “if” part to obtain the result shown in the bottom image?

You can remove the conditions by extracting the first element of the ConditionalExpression:
Solve[ForAll[{x, y},
Equivalent[
m (x/a) + n (y/b) ==
1, ((t1 - t2) (b (-1 + t1 t2) x +
a (b + b t1 t2 - (t1 + t2) y)))/((1 + t1^2) (1 + t2^2)) ==
0]], {m, n}, Reals] /.
ConditionalExpression -> (#1 &) // Simplify
mroge02
(Michael Rogers)
4
Normal[] converts ConditionalExpression[value, condition] to its un-Conditional-ized value:
Normal@Solve[...]